1. Field of the Invention
This invention relates to a sensitive element for use in a strain sensor and to a pressure sensor using such an element.
By strain sensor is meant a device capable of measuring a force to which it is subjected or a device capable of measuring a mechanical strain or a pressure.
2. Prior Art
For measuring atmospheric pressure and pressure changes by means of portable apparatus, such as an altimeter, several phenomena may be made use of. Generally, an evacuated capsule having parallel walls consisting of highly elastic diaphragms is used as a detector or sensitive element. In an altimeter of the usual kind, the deformation or bending to which the bottom of the capsule is subjected by the outer pressure is amplified and mechanically converted into a rotary motion of a needle via a gear-train.
Further, piezoresistive sensors are at present found on the market that consist of a silicon crystal diaphragm in which resistances are formed by diffusion. When a pressure difference exists between the two surfaces of the diaphragm, the resulting deformation causes the resistance to vary. Generally, four resistances are made at the same time so as to produce a temperature-compensated measuring circuit in the form of a bridge, with all resistances having the same temperature coefficient.
All of these detectors are relatively costly since they require very careful individual adjustment. Also, the measured physical magnitude is converted into another magnitude which is analogical: angular movement or change in resistance. In a recent publication (IEEE Transactions on Industrial Electronics and Control Instrumentation, Volume IECI 25, Number 1, February 1978, pages 29 to 38), a proposal was made to design pressure-measuring capsules with a resonant structure. The sensitive diaphragm forms a vibrating membrane and use is made of the resonant frequency of the capsule which is coupled to an electronic oscillator, said frequency varying with the outside pressure applied to the capsule. However, the construction of such a resonant capsule is very tricky and is not suited to economic production on a very large scale.
It is also known from European published specification No. 50307 to produce a force sensor from a quartz resonator. Such a sensor could in principle also measure pressures. But such a sensor fails to solve the problem caused by temperature changes.